Process for curing acrylic polymers

ABSTRACT

Acrylic polymers are typically difficult to cure unless they contain specific curing sites, such as carbon-carbon double bonds. This invention offers a technique to rapidly cure acrylic polymers containing nitrile groups that do not possess typical curing sites, such as carbon-carbon double bonds. By utilizing the process of this invention, heat resistant thermoplastic vulcanizates with high-resistance to solvents can easily be prepared. The present invention more specifically discloses a process for curing an acrylic polymer which comprises distributing an aromatic compound having at least two nitrile oxide groups bonded thereto throughout the acrylic polymer, wherein the acrylic polymer contains nitrile groups, and wherein a bulky group is in at least one ortho position to the nitrile oxide group and wherein the bulky group is a moiety which is larger than a hydrogen atom.

FIELD OF THE INVENTION

This invention relates generally to the curing of acrylic polymers. It more specifically relates to the curing of saturated acrylic polymers containing nitrile groups with stable aromatic dinitrile oxide compounds.

BACKGROUND OF THE INVENTION

Most nitrile oxides (RCNO) are little known, short-lived reactive species that are structurally isomeric with isocyanates (RNCO) and cyanates (ROCN). Being reactive, they readily undergo 1,3-dipolar additions with a great variety of multiple bond functional groups. The relative decreasing reactivity of nitrile oxides with these multiple bonds is roughly: C═S, N═N, P(V)═C>C═P(III), C═As, C═C, C═N, C═Se, B═N>C═P, C═C>P(V)═N, C═N>C═O. In the absence of these functional groups, most nitrile oxides will readily dimerize to furoxans (1,2,5-oxadiazole-2-oxides).

Nitrile oxides are mild oxidants and will liberate iodine from solutions of potassium iodide. The nitrile oxide function imposes the same type of solubility characteristics on a molecule that a cyano group does. Aromatic nitrile oxides, especially those where the nitrile oxide is flanked by at least one and preferably two ortho groups have been found to be stable compounds. For example, 2,4,6-Trimethylbenzonitrile oxide is a stable crystalline solid (m.p. 105° C.-108° C.) with two characteristic strong IR absorptions at 2290 cm⁻¹ (—C═N) and 1334 cm⁻¹ (—N≡O). In the ¹³C NMR, the carbon in the —CNO group is found at 35.7 ppm compared with 117.5 ppm for the carbon in the corresponding nitrile compound. This implies that the carbon atom carries considerable negative charge whereas the oxygen bears a positive charge. This highly dipolar character explains their high reactivity toward multiple bonds, see K B G Torssell, “Nitrile Oxides, Nitrones and Nitronates in Organic Synthesis,” New York: VCH Publ, 1988.

The history of using polyfunctional nitrile oxides or their precursors as crosslinking agents for unsaturated polymers appears to go back to at least 1968 when chemists from Hercules were granted U.S. Pat. No. 3,390,204 for this application. In most cases, the polyfunctional nitrile oxide itself was too unstable to use directly. In these cases, the polyfunctional hydroximoyl halides were used as stable precursors. When mixed with unsaturated elastomers and then exposed to a base such as triethylamine, the hydroximoyl halide function immediately dehydrohalogenates to produce the nitrile oxide group which then rapidly crosslinks the elastomer. The resulting unfilled crosslinked elastomers from styrene-butadiene rubber (SBR), polybutadiene rubber (PBd) and natural rubber (NR) were described as hard, tough and substantially insoluble in chloroform. The crosslinking system was also noted not to be affected by air or moisture. Oddly, this patent makes no mention of any actual cured physical properties or potential applications.

Because no practical system evolved from this work, we can assume it had serious drawbacks either in process control, cost, toxicity and/or ultimate physical properties. Obviously, a two-component system requiring both a hydroximoyl halide and an organic base is not desirable, whereas the use of polynitrile oxide compounds alone is limited only to those nitrile oxides whose stability is sufficient to allow their dispersion and reaction with rubber in preference to self-dimerization. As previously mentioned, very few stable nitrile oxides are known even today. What is known about this class of chemicals is that aromatic nitrile oxides with one or two ortho substituents have enhanced chemical stability with regard to dimerization resistance. Unfortunately, the prime precursors for such nitrile oxides are sterically hindered aromatic dialdehydes. Dialdehydes of this type are relatively difficult to prepare in good yields and purity and this lack of a good synthetic method probably accounts for the field's lack of development.

In 1989, the situation began to change when Russian chemists developed a novel approach to the synthesis of sterically hindered aromatic dialdehydes (see Leonid I Belen'kii et al, Tetrahedron, Vol. 49, No 16, pages 3397-3404, 1993, and Russian Patent SU 4,750,502). With this new technique, aromatic hydrocarbons such as mesitylene or durene could be converted (in several steps) into dialdehydes in yields between 58 to 71 percent (based on hydrocarbon). This was a substantial improvement over the known technique of oxidizing bis-(hydroxymethyl) mesitylene to the dialdehyde with lead tetraacetate described by Christoph Grundmann and Reinhard Richter, “Preparation of the Dinitrile Oxide of Mesitylene,” The Journal of Organic Chemistry, 33, 476 (1968).

Within a few years, another Russian group revisited the topic of rubber curing with dinitrile oxides. Only this time, they now had the dinitrile oxide of mesitylene (MDNO) readily available because of the new dialdehyde synthesis (see V V Boiko, N D Malaya and L M Klimenko, “Rheological properties of solutions of diene elastomers with the mesitylene dinitrile oxide,” International Polymer Science and Technology, Vol. 20, No 10, T51, 1993). In their initial studies, they investigated the change in viscosity of various elastomer solutions as a function of time, temperature and MDNO concentration. Polymer solutions evaluated were cis-polyisoprene, polybutadiene, SBR and NBR. The relative rate of gelation was PBd>SBR≈NBR >IR. With PBd and two parts of MDNO, solution gelation took place in 1.5 hours at 25° C.; 0.5 hours at 40° C. From this work, they concluded that MDNO could be used as an efficient low temperature vulcanizing agent for a wide variety of diene elastomers. More recent work by the same Russian group using MDNO and various diene elastomers showed the same order of reactivity as measured by Mooney viscosity increases during mixing (see V V Boiko and I V Grinev, “Influence of MDNO/Processing Elastomers,” International Polymer Science and Technology, Vol. 22, No 7, T/21, 1995).

The first practical application of this technology appears to be described in Russian Patent SU 1,825,829-Al to the Ukrainian Textile Industry Research Institute. In this patent, polymethyl-vinyl-siloxane rubber having an M.W. of 500,000 and a molar olefin content of 0.45-0.55 percent was crosslinked with MDNO in ethyl acetate on a fabric.

The crosslinked fabric treatment produced a durable water and dirt repellent finish. Russian Patent SU 1,824,389-Al has also been issued to the Ukrainian Textile Industry Research Institute for the synthesis of 2,4,5-trimethylbenzene-1,3-dialdehyde as an intermediate to the corresponding dinitrile oxide as a low temperature hardener. Russian Patent 2,042,664-C1 describes a new synthesis for dialkylbenzene dinitrile oxides and demonstrated their utility for curing rubbers with low levels of unsaturation at low temperature. Whereas, MDNO has its nitrile oxide groups each flanked by two methyl groups, the new dinitrile oxide compounds have each —CNO group flanked by only one alkyl group. It is not clear, however, whether or not this structural difference would result in different rates of cure since a comparison control with MDNO was not included in the cured rubber physical property data. Nevertheless, a comparison of the room temperature cured properties of several low unsaturation polymers with the same polymers cured with conventional high temperature sulfur or peroxide systems, showed remarkable similarity.

The starting materials for this new synthesis are meta or para-dimethyl- or meta or para-diethylbenzene. The hydrocarbons are treated with specific molar ratios of aqueous formaldehyde solution, hydrochloric acid, sulfuric acid and acetic acid at 70° C.-85° C. to prepare bis-(chloromethyl)-dialkylbenzenes (see Milton J Rhoad and Paul J Flory, “Preparation of Bischloromethylmesitylene,” Journal of the American Chemical Society, 72, 2216 (1950)). After cooling to 15°C.-25° C., crystals of the bis-chloromethyl compounds are then filtered off, washed with water and dried. In the next step, the bis-chloromethyl compounds are treated with an aqueous solution of hexamethylenetetramine in acetic acid for 4 hours at 95° C.-100° C. The organic products (dialdehydes) were then extracted from the reaction mixture with carbon tetrachloride. After water-washing the carbon tetrachloride solution, an aqueous solution of hydroxylamine hydrochloride was added followed by the addition of an aqueous NaOH solution to generate free hydroxylamine.

The organic layer was then separated after about 1 hour at 25° C.-35° C. The aqueous layer was cooled to 20° C. and neutralized to pH 7 with HCl to precipitate the dioxime. The dioxime is filtered off, washed with water and dried.

Treatment of the dioxime with acidified sodium hypochlorite solution at −10° C. to 0° C. gave the crystalline dinitrile oxide compound in very good yields.

Although this synthesis is a great improvement over previous methods, it does have several drawbacks such as the use of a chlorinated solvent and multiple extractions. Additionally, the hexamethylenetetramine/acetic acid method for conversion of the chloromethyl group into an aldehyde is not synthetically useful for the conversion of more highly hindered halomethyl groups such as those found in halomethylated mesitylene.

A W van der Made and R H van der Made, “Preparation of Bromomethylaromatic Compounds,” The Journal of Organic Chemistry, 58, 1262 (1993) reports that mesitylene and other structurally similar aromatic compounds can be efficiently bis-bromomethylated in high yields using readily available reagents. This technique allows more efficient access to bis-bromomethyl compounds which can subsequently be converted into dinitrile oxides by employing prior art techniques.

SUMMARY OF THE INVENTION

Acrylic polymers are typically sticky thermoplastic materials prior to being crosslinked. After being crosslinked, the acrylic polymer becomes more rubbery in nature. However, acrylic polymers are typically difficult to cure or crosslink unless they contain carbon-carbon double bonds. This invention offers a technique to rapidly cure acrylic polymers containing nitrile groups that do not possess typical curing sites, such as carbon-carbon double bonds. By utilizing the process of this invention, heat-resistant thermoplastic vulcanizates with high resistance to solvents can easily be prepared. The Compression resistance of the acrylic polymer is also increased after it is cured. Additionally, acrylic polymers that are cured by utilizing the techniques of this invention do not require a post-curing treatment to complete the vulcanization process.

Stable aryl dinitrile oxides, such as mesitylene dinitrile oxide, are used in the process of this invention to cure acrylic polymers. The stable dinitrile oxide is simply distributed into the acrylic polymer. After the stable dinitrile oxide is mixed into the acrylic polymer, crosslinking occurs at a low temperature, such as room temperature (about 20° C.). Thus, the utilization of stable dinitrile oxides as crosslinking agent also offers the advantage of low temperature curing. This is beneficial because energy requirements are reduced and because thermal polymer degradation is eliminated.

The present invention discloses a process for curing an acrylic polymer which comprises distributing an aromatic compound having at least two nitrile oxide groups bonded thereto throughout the acrylic polymer, wherein the acrylic polymer contains nitrile groups, and wherein a bulky group is in at least one ortho position to the nitrile oxide group and wherein the bulky group is a moiety which is larger than a hydrogen atom. As a general rule, from about 0.1 phr (parts per hundred parts of the acrylic polymer) to about 5 phr of the stable dinitrile oxide compound will be blended into the acrylic polymer.

This invention further discloses a crosslinked acrylic polymer which is comprised of polymer chains of an acrylic polymer, wherein said polymer chains of the acrylic polymer are crosslinked through crosslink sites which are derived from a stable dinitrile oxide compound. The crosslink sites in the crosslinked acrylic polymer will typically be of the formula:

wherein Φ represents an aromatic structure having a bulky group in at least one ortho position to the 5-membered rings, wherein the aromatic structure has a ratio of hydrogen atoms to carbon atoms of less than 1:1 and wherein the bulky group is a moiety which is larger than a hydrogen atom.

DETAILED DESCRIPTION OF THE INVENTION

The stabilized aryl dinitrile oxides that are utilized in the process of this invention are made by a four-step process. In the first step of the process, two halomethyl groups are introduced onto a substituted aromatic compound. The substituted aromatic compound will contain at least one alkyl substituent group, aryl substituent group, fused aryl substituent group, alkaryl substituent group, halogen substituent group, or alkoxy substituent group. In this step, the halomethyl groups are introduced onto positions that are ortho to at least one of the substituent groups on the substituted aromatic compound. As a specific example, in the case of naphthalene, the halomethyl groups are introduced onto a position that is ortho to the fused aryl group (benzene ring) as shown below:

wherein X represents a halogen atom.

A wide variety of substituted aromatic compounds can be utilized as the starting material. These substituted aromatic compounds will contain at least one alkyl, aryl, fused aryl, alkaryl, halogen, or alkoxy substituent group. Some representative examples of substituted aromatic compounds that can be used include: 1,3,5-trimethylbenzene (mesitylene), 1,2,4,5-tetramethylbenzene (durene), toluene, o-xylene, m-xylene, p-xylene, pseudocumene, isodurene, prehnitene, 1,3-dimethyl-5-t-butylbenzene, 1,3,5-triethylbenzene, 1,3,5-tripropylbenzene, naphthalene, anthracene, α-methylnaphthalene, β-methylnaphthalene, phenanthrene, 1,2-benzanthracene, p-dichlorobenzene, o-chlorotoluene, p-chlorotoluene, p-bromotoluene, bromodurene, bromoisodurene, bromoprehnitene, bromomesitylene, 1-chloro-1-mesitylethylene, p-bromoethylbenzene, 1-chloro-1-mesitylpropene, α-chloroisodurene, 1,4-diethoxybenzene, 1,4-dipropoxybenzene, 1,4-dibutoxybenzene, 1,4-diamyloxybenzene, 2,5-dimethoxytoluene, 2,5-dimethoxyoctylbenzene, 2,5-dimethoxybromobenzene, 3,-chloromethyl-1,2-dimethoxybenzene, 4-(β-chloropropyl)-1,2-dimethoxybenzene, 2,3,4-trimethoxybromobenzene, trimethoxy-p-xylene, 5-propyl-1:3-benzodioxole, and the like. Mesitylene and durene are preferred substituted aromatic compounds.

This halomethylation reaction can be carried out by reacting the substituted aromatic compound with a mixture of acetic acid, hydrogen bromide and paraformaldehyde at a temperature which is within the range of about 50° C. to about 120° C. For instance, one mole of the substituted aromatic compound (mesitylene) can be reacted with about two moles of the hydrogen bromide and about two moles of the paraformaldehyde. It is typically preferred from this reaction to be carried out at a temperature which is within the range of about 80° C to about 95° C.

The halomethylation reaction can, in the alternative, be carried out by utilizing the procedure described by Milton J Rhoad and Paul J Flory, “The Synthesis of Polymeric Ethers,” Journal of the American Chemical Society, Vol. 72, page 2216 (1950), the teachings of which are incorporated herein by reference in their entirety. In this procedure, 50 grams (0.37 mole) of durene (melting point of 79° C. to 81° C.) dissolved in 200 ml (1.5 moles) of 40 percent aqueous formaldehyde and 100 ml of concentrated hydrochloric acid were heated with stirring on a steam bath while a slow stream of hydrogen chloride gas was bubbled through the mixture. After six hours, the oil layer was separated while hot and set aside to cool. The fine white needles of crude product which deposited where collected, leaving the intermediate monochloromethyldurene in solution. The liquor was treated with a fresh formaldehyde-hydrochloric acid mixture as described above and an additional amount of crude product was obtained. A total of six such treatments of the original durene solution yielded 69 grams (80 percent) of crude bis-(chloromethyl)-durene. A single recrystallization from benzene gave 58 grams (67 percent). The product obtained had a melting point of 193° C.-194° C.

In the second step of the process used to synthesize the stable aryl dinitrile oxide, the ortho halomethylated-substituted aromatic compound is converted into an ortho-substituted aromatic aldehyde by reacting the ortho halomethylated-substituted aromatic compound with a salt of 2-nitropropane. The salt of 2-nitropropane will typically be a sodium or potassium salt. This step is carried out, utilizing a lower alcohol as the solvent. The lower alcohol will typically contain from 1 to 5 carbon atoms. Some representative examples of alcohols that can be employed as the solvent include methanol, ethanol, n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, isobutyl alcohol, t-butyl alcohol and n-pentyl alcohol. This reaction step will typically be carried out at a temperature that is within the range of about 20° C. to about 100° C. It is normally preferred for this reaction to be conducted at a temperature which is within the range of about 30° C. to 90° C., with temperatures in the range of 40° C to 85° C being most preferred.

In the third step of the process used in making the stable aryl dinitrile oxide compound, the ortho-substituted aromatic aldehyde is converted into an ortho-substituted aromatic oxime by reacting the ortho-substituted aromatic aldehyde with hydroxylamine. This reaction step can be carried out in the same reaction vessel in which the ortho-substituted aromatic aldehyde was synthesized or the ortho-substituted aromatic aldehyde can be isolated and reacted with the hydroxylamine in a separate reaction vessel. This reaction step will typically be carried out at a temperature which is within the range of about 20° C. to 100° C. Reaction temperatures in the range of 30° C. to 70° C. are preferred, with temperatures within the range of 35° C to 55° C being most preferred.

In the final step of the synthesis of the aryl dinitrile oxide compound, the ortho-substituted aromatic oxime is converted into an ortho-substituted aromatic nitrile oxide or ortho-substituted aromatic dinitrile oxide by reacting it with an aqueous sodium hypochlorite (bleach) solution. It is normally preferred to utilize a solution that contains from about 4 weight percent to about 6 weight percent sodium hypochlorite. This reaction step will typically be carried out at a temperature which is within the range of about −5° C. to 20° C.

As has been explained, this technique can be utilized to convert a wide variety of substituted aromatic compounds into stable aryl dinitrile oxide compounds, such as ortho-substituted aryl mononitrile oxides and ortho-substituted aryl dinitrile oxides. The following reaction is representative of conversions of a substituted aromatic compound into a stable nitrile oxide that can be carried out utilizing the techniques of this invention:

wherein each R¹ is independently C₁-C₁₂-alkyl, F, Cl, Br, I, O—C₁-C₁₂-alkyl or S-C₁-C₁₂-alkyl; each R⁰ is a substituent that does not spontaneously react with the nitrile oxide group; each n′ is independently 0, 1 or 2; n″ is an integer greater than 1; each X′ is independently a bond or a connecting group; and Y′ is a polyvalent radical containing an ether, ester, amide, amine, carbonate, ketone, urethane, arylene or thioether moiety; or each X′ and Y′ together are a bond connecting the benzene rings.

The following reactions are also representative of the conversion of various types of substituted aromatic compounds into stable aryl dinitrile oxides:

where R¹, R² R³ and R⁴ are each independently H, R, halo, SH, SR, SOR, SO₂R, hydroxy, or OR, with the proviso that R represents an alkyl group, an aryl group or an alkaryl group, with the proviso that at least one of R¹, R², R³ and R⁴ that is adjacent to a nitrile oxide group is not H; R⁵, R^(6,) R⁷ and R⁸ are each independently H, R, halo, S-H, SR, SOR, SO₂R, hydroxy or OR, wherein R is a C₁-C₁₂ linear, branched or cyclic alkyl group, preferably a C₁-C₄ linear or branched alkyl group, more preferably ethyl or methyl; or R5 and R⁶ or R⁷ and R⁸, together with the carbon atoms to which they are attached, form a benzene ring, wherein at least one of R⁵ or R⁷ is not H, and at least one of R⁶ or R⁸ is not H; i is 2 or 3; m and n are each 0, 1 or 2 and n+m≧2, preferably 2 or 3.

The following reactions are also representative of the conversions of various types of substituted aromatic compounds into stable aryl dinitrile oxides:

where R⁹, R¹⁰,R¹¹ and R¹² are each independently H atoms, halogen atoms, —SH groups, —SR groups, —SOR groups, —R groups, —SO₂R groups, hydroxy groups or —OR groups with the proviso that at least one of R⁹ and R¹¹ is not hydrogen when a nitrile oxide group is adjacent to both R⁹ and R¹¹ and at least one of R¹⁰ and R¹² is not hydrogen when a nitrile oxide group is adjacent to both R¹⁰ and R¹²; m, p and r are each 0, 1 or 2 and p+r ≧2; X is CH₂, C(R)₂, carbonyl, O, S, SO, SO_(2,) NH, SO₂NH, SO₂NR or NR; t and u are each 0, 1, 2 or 3; and t+u≧2; Y is a bond, CH₂, C(R)₂, carbonyl, O, S, SO, SO ₂, NH, NR, 9,9′ -fluoreno or phenylene.

The following reactions are also representative of the conversions of various types of substituted aromatic compounds into stable nitrile oxides:

The aryl dinitrile oxide compounds that are used in curing acrylic polymers in accordance with this invention are aromatic compounds having at least two nitrile oxide groups bonded thereto, wherein a bulky group is in at least one ortho position to the nitrile oxide group. The bulky group will be larger than a hydrogen atom and will preferably be larger than a fluorine group. It is preferred for two bulky groups to be in ortho positions to the nitrile oxide group.

The stable aryl dinitrile oxide compound will frequently be of the formula:

wherein R¹, R² and R³ can be the same or different and are selected from alkyl groups, aryl groups, aryl groups, alkaryl groups, halogen atoms, and alkoxy groups. As has been explained, a fused aryl group can also be an ortho position to the nitrile oxide groups. Examples of such aryl dinitrile oxide compounds include the following:

wherein Z¹, Z², Z³, Z⁴, Z⁵ and Z⁶ represent nitrile oxide groups or hydrogen atoms, with the proviso that two of the members selected from the group consisting of Z¹, Z², Z³, Z⁴,Z ⁵ and Z⁶ represent nitrile oxide groups, and with the proviso that the other members selected from the group consisting of Z¹, Z², Z³, Z⁴, Z⁵ and Z6 represent hydrogen atoms.

Virtually any type of acrylic polymer that contains nitrile groups can be cured by using the process of this invention. These acrylic polymers will typically be comprised of repeat units that are derived from a nitrile group containing monomer, such as acrylonitrile, and at least one alkyl acrylate monomer and/or at least one alkyl methacrylate monomer. The alkyl acrylate monomers that can be used generally have the structural formula:

wherein R represents an alkyl group containing from 1 to about 10 carbon atoms. The alkyl group in such alkyl acrylate monomers will preferably contain from 2 to 8 carbon atoms. Accordingly, ethyl acrylate, propyl acrylate, butyl acrylate, pentyl acrylate, hexyl acrylate and 2-ethyl hexyl acrylate are preferred alkyl acrylate monomers. The alkyl groups in such alkyl acrylate monomers can be straight chained or branched. Thus, normal-propyl acrylate, isopropyl acrylate, normal-butyl acrylate or tertiary-butyl acrylate can be employed.

The alkyl methacrylate monomers that can be used are of the structural formula:

wherein R represents an alkyl group containing from 1 to 10 carbon atoms. The alkyl groups in such alkyl methacrylate monomers can be straight chained or branched. Methyl methacrylate, ethyl methacrylate, propyl methacrylate, isobutyl methacrylate and n-butyl methacrylate are representative examples of alkyl methacrylates that can be used. The alkyl group in the alkyl methacrylate monomer will preferably contain from 1 to 4 carbon atoms. The acrylic polymer can also contain repeat units that are derived from one or more vinyl aromatic monomers. Any vinyl aromatic monomer that will copolymerize with acrylic monomers can be used. Such vinyl aromatic monomers typically contain from 8 to about 20 carbon atoms. Usually, the vinyl aromatic monomer will contain from 8 to 14 carbon atoms. Some representative examples of vinyl aromatic monomers that can be used include styrene, para-tertiary-butyl styrene, 1-vinyl naphthalene, 2-vinyl naphthalene, 3-methyl styrene, 4-propyl styrene, 4-cyclohexyl styrene, 4-dodecyl styrene, 2-ethyl-4-benzyl styrene 4-(phenylbutyl)styrene, and the like.

Acrylonitrile containing acrylic polymers that can be cured by the process of this invention are sold by The Goodyear Tire & Rubber Company as Sunigum® rubber. Acrylic polymers of this type are comprised of repeat units that are derived from (a) butyl acrylate, (b) at least one acrylate monomer selected from the group consisting of methyl methacrylate, ethyl methacrylate, methyl acrylate and ethyl acrylate, (c) acrylonitrile and (d) a crosslinking agent. The crosslinking agents utilized in acrylate polymers of this type are typically difunctional acrylates, difunctional methacrylates, trifunctional acrylates, trifunctional methacrylates or divinylbenzene. Another crosslinking agent that has proven to be highly useful is 1,4-butanediol dimethacrylate.

Acrylate polymers that contain nitrile groups can be cured by simply blending the stable dinitrile oxide into the acrylic polymer. This can be done using any suitable mixing means that will distribute the stable aryl dinitrile oxide compound throughout the acrylic polymer. The stable aryl dinitrile oxide compound then reacts with the acrylic polymer to form crosslinks. The crosslink sites in the crosslinked acrylic polymer will typically be of the formula:

wherein Φ represents an aromatic structure having a bulky group in at least one ortho position to the 5-membered rings, wherein the aromatic structure has a ratio of hydrogen atoms to carbon atoms of less than 1:1 and wherein the bulky group is a moiety which is larger than a hydrogen atom.

This invention is illustrated by the following examples that are merely for the purpose of illustration and are not to be regarded as limiting the scope of the invention or the manner in which it can be practiced. Unless specifically indicated otherwise, all parts and percentages are given by weight.

EXAMPLE 1

In this experiment, mesitylene dinitrile oxide (MDNO) was synthesized utilizing the technique of this invention. In the first step of the procedure used, mesitylene was halomethylated to produce 1,3-bis(bromomethyl) -2,4,6-trimethylbenzene. This reaction can be depicted as follows:

In conducting this reaction, a 2-liter three-necked round-bottomed flask equipped with a mechanical stirrer, condenser and pot thermometer was charged with 96.0 g (0.80 moles) of 98 percent pure mesitylene (Aldrich), 51.79 g (ca. 1.6 moles) of 95 percent pure paraformaldehyde (Aldrich), 200 ml of glacial acetic acid and 433 g (320 ml) (1.6 moles) of 30 percent hydrogen bromide (HBr) in acetic acid solution (Aldrich). The mixture was slowly heated with stirring to 90° C.-95° C. and held at this temperature for 7 hours.

After several hours heating, product dibromide compound begins to precipitate from the reaction mixture. The mixture was allowed to stand at room temperature overnight. The crystalline mass was then broken up and stirred to form a uniform slurry before adding 1 liter of tap water to complete the precipitation. The product was filtered, washed with water and air-dried in a hood to give 287.6 g of crude product (theory 244.64 g). G.C. analysis (area percent) of the crude product using a 30-meter DB-1 capillary column and a program of 100° C.-270° C. at 15° C./minute showed the material to consist of 93.4 percent dibromide (9.28 minutes retention time), 1.5 percent monobromide (5.46 minutes) and 1.3 percent tribromide (12.65 minutes). This material was used without purification in the next step of the process.

The next step of the process involved the in-situ preparation of the Bisoxime of 2,4,6-Trimethylisophthaldehyde from 1,3-Bis(Bromomethyl)-2,4,6-Trimethylbenzene by reactions that can be depicted as follows:

In conducting these reactions, a 5-liter three-necked round-bottomed flask equipped with a mechanical stirrer, condenser, pot thermometer and nitrogen inlet tube was charged with 118.56 g of 85 percent KOH pellets (1.8 moles), 1800 ml of isopropanol and 160.4 g (1.8 moles) of 2-nitropropane (Aldrich). This mixture was then stirred under a slow nitrogen bleed until the potassium hydroxide (KOH) had essentially completely dissolved. This process takes approximately 30-45 minutes. During this period, the solution temperature gradually increases to about 38° C.-40° C. At this point, 287.6 g (ca. 0.80 moles) of the crude dibromide was added to the solution all at once. The temperature of the mixture gradually increased to about 75° C.-78° C. as the reaction proceeded before subsiding. After about 1.5 hours, the temperature had fallen to about 35° C.-40° C. G.C. analysis of the mixture using the same column and conditions used for the dibromide showed it to be completely consumed. The major product peak for 2,4,6-Trimethylisophthaldehyde appears at 6.87 minutes retention time. Assuming complete conversion of dibromide to dialdehyde, 112.2 g of 50 percent aqueous hydroxylamine solution (ca. 1.7 moles) was then added to the mixture (at 45° C.) over about 5 minutes. The temperature of the mixture increased to about 51° C. After 1 hour, G.C. analysis showed no aldehyde peak at 6.87 minutes. The solution was then filtered to remove salts, such as potassium bromide (KBr). Salt cake was washed with 200 ml isopropanol and the filtrates combined. The alcohol solution was then poured into a large excess of stirred cold water to precipitate the crude oxime. After filtration and drying, 128.1 g was isolated. This represented a 77.7 percent yield.

In the final step of this procedure, Mesitylene Dinitrile Oxide was prepared by the reaction:

A mechanical stirrer with a stainless steel agitator blade was set up in a 5-liter stainless steel beaker. With stirring, 115.25 g (ca. 0.559 moles) of the crude bis-oxime was then dissolved in 2500 ml of isopropanol in the beaker. The beaker was placed in an ice water bath and the solution cooled to 10° C.-15° C. Then, 1825 g (ca. 1.225 moles) of commercial bleach (ca. 5 percent NaCl0) was slowly added dropwise to the stirred oxime solution over a 6-hour period. As the product MDNO formed, it precipitated from the isopropanol mixture. Ten minutes after all the 5 percent sodium hypochlorite has been added, the mixture was diluted with 4 liters of water and then filtered to remove the crude product. The product was air-dried to yield 98.0 g of a white solid having a melting point of 137° C.-140° C. The FTIR spectrum of this material shows a very sharp distinctive absorption band for the —CNO functionality at 2296 cm⁻¹.

EXAMPLE 2

In this experiment, dinitrile oxides were prepared from a commercial source of divinylbenzene that contained m-divinyl-benzene, p-divinylbenzene, 1-ethyl-3-vinyl-benzene and 1-ethyl-4-vinylbenzene.

In the first step of the process, meta/para diethylbenzenes were prepared by the reaction:

In carrying out this reaction, a 500-ml Parr hydrogenation bottle was charged with 0.5 g of 5 percent Pd/C catalyst followed by 200 ml of ethanol and 50.0 g (ca. 0.423 moles olefin) of the 55 percent commercial divinylbenzene (DVB). Then, the bottle was attached to Parr apparatus and briefly evacuated to remove air. Repressure to 45-50 psig with hydrogen. Upon shaking, the temperature of the reaction mixture increased from 75° C. to 17° F. over 25 minutes. Hydrogenation abruptly stopped after 48 pounds of hydrogen had been taken up. G.C. area percent analysis of starting material using a 30-meter DB-1 and 70° C. isothermal program indicated that the 55 percent DVB material actually consists of 32.7 percent 1-vinyl-3-ethylbenzene, 10.65 percent 1-vinyl-4-ethylbenzene, 38.15 percent 1,3-divinylbenzene and 17.25 percent 1,4-divinylbenzene. After hydrogenation, the product consists of 71.52 percent 1,3-diethylbenzene and 28.3 percent 1,4-diethylbenzene. After removal of the ethanol solvent, the crude product was used as raw material for the following chloromethylation reaction.

Then, Mixed Isomer Bis-(Chloromethyl)-Diethylbenzene was synthesized according to the reaction:

In conducting this reaction, a 1-liter three-necked round-bottomed flask equipped with mechanical stirrer, pot thermometer, condenser, dropping funnel and heating mantel was charged with 67.4 g (0.784 moles) of 35 percent aqueous formaldehyde solution, 81.8 g (69.8 ml) (0.784 moles) of conc. HCl, 33.0 g (31.4 ml) (0.55 moles) of glacial acetic acid and 35.2 g (0.2627 moles) of m/p-diethylbenzene. To this mixture, add dropwise over 30 minutes 180.4 g (100 ml) or (1.84 moles) of conc sulfuric acid at 70° C.-85° C. The temperature was then held constant for 4 hours. Analysis of mixture by G.C. showed 10 percent diethylbenzene, 55 percent monochloromethylated isomers and 28 percent dichloromethylated isomers at this stage. An additional 34.9 ml of conc HCl and 15 g of 35 percent formaldehyde solution were then added and the mixture refluxed for an additional 3 hours. Analysis then showed 29 percent mono and 65 percent dichloromethylated products. After standing at room temperature overnight, a crystalline solid precipitated and was isolated by filtration, washed with hexane and dried to give 49.05 g product. G.C. analysis showed the material to be 93.1 percent dichloromethylated diethylbenzenes consisting mostly of one isomer.

Then, Bis-oximes of Diethylterephthalaldehyde were synthesized in situ from Bis-(Chloromethyl)-Diethylbenzenes according to the following reactions:

In the procedure used to carry out this reaction, a 1-liter three-necked round-bottomed flask equipped with a mechanical stirrer, pot thermometer, condenser and nitrogen tube inlet was charged with 30.3 g (0.46 moles) of 85 percent KOH pellets, 450 ml isopropanol and 41.0 g (0.46 moles) of 2 nitropropane. The mixture was stirred under a slow nitrogen bleed for about 30 minutes until all the KOH had dissolved. The solution was warmed to about 50° C. and 49.0 g (0.213 moles) of the mixed isomers of bis-(chloromethyl)-diethylbenzene were added all at once. The temperature gradually increased to 68° C. After 1 hour, G.C. analysis indicated that no starting material remained. Two new aldehyde peaks were observed with retention times of 6.80 and 6.95 minutes with the DB-1 column and a program of 100° C.-270° C. at 15° C./min. At 55° C., 30.36 g (0.46 moles) of 50 percent aqueous hydroxylamine was added over about 10 minutes. The temperature increased to 67° C. After 30 minutes, G.C. analysis indicated that all the aldehyde isomers had been consumed. The warm reaction mixture was poured into excess cold water to solidify the product. The crude product was filtered, washed with water and air-dried to give 40.25 g of flesh-colored solid. Theoretical yield should be 46.86 g. The crude yield was 85.9 percent.

Dinitrile Oxides were then prepared from the Mixed Isomers of Diethylisophthaloaldehyde Dioximes. In the procedure used, mechanical stirrer with a stainless steel agitator blade was set up in a 2-liter stainless steel beaker. Then, 40.25 g (ca. 0.1829 moles) of the crude bis-oxime was then dissolved in 1000 ml of isopropanol in the beaker with stirring. The beaker was placed in an ice water bath and the solution cooled to 10° C.-15° C. Over a 2-hour period, 597 g (ca. 0.40 moles) of commercial bleach (ca. 5 percent NaCl0) was then slowly added dropwise to the stirred oxime solution. As the product dinitrile oxide forms, it precipitates from the isopropanol mixture. Ten minutes after all the 5 percent sodium hypochlorite has been added, the mixture was diluted with 1 liter of water and then filtered to remove the crude product.

The product was air-dried to yield 31.6 g of a yellow-gold solid. The FTIR spectrum of this material shows a very sharp distinctive absorption band for the —CNO functionality at 2285 cm⁻¹. Crude yield was determined to be 79.2 percent.

EXAMPLE 3

In this experiment, dinitrile oxides were synthesized from 1,2,4-Trimethylbenzene. In the first step of the experiment, bis- (bromomethyl) isomers of 1,2,4-trimethylbenzene were synthesized according to the reaction:

In the procedure used, a 1-liter three-necked round-bottomed flask equipped with a mechanical stirrer, condenser and pot thermometer was charged with 24.0 g (0.20 moles) of 1,2,4-trimethylbenzene (Aldrich), 20.65 g (ca. 0.655 moles) of 95 percent pure paraformaldehyde (Aldrich) and 178 g (131.5 ml) (0.655 moles) of 30 percent HBr in acetic acid solution (Aldrich). The mixture was slowly heated with stirring to ca 80° C., where a mild exotherm was noted. Heating was discontinued while temperature rose to 85° C.-90° C. After exotherm subsided, temperature was increased to 108° C.-110° C. and held there for 6 hours.

The mixture was allowed to stand at room temperature overnight. The crystalline solid was then filtered off and washed with hexane and air-dried. G.C. analysis of the crude product using a 30-meter DB-1 capillary column and a program of 100° C.-270° C. at 15° C./minute showed the material to consist of 84.0 percent dibromides (8.93 minute and 9.41 minute retention time), 6.0 percent monobromide (5.66 minutes) and 5.7 percent tribromide (12.36 minutes). This material was used without further purification in the next step of the process.

In the next step of the process, Bis-oximes of 1,2,4-Trimethylisophthalaldehyde were synthesized from Bis-(Bromomethyl)-1,2,4-trimethylbenzenes according to the following reactions:

In the procedure used, a 1-liter three-necked round-bottomed flask equipped with a mechanical stirrer, pot thermometer, condenser and nitrogen tube inlet was charged with 28.33 g (0.43 moles) of 85 percent KOH pellets, 400 ml isopropanol and 39.2 g (0.44 moles) of 2 nitropropane. The mixture was stirred under a slow nitrogen bleed for about 30 minutes until all the KOH had dissolved. The solution was warmed to about 50° C. and 65.5 g (ca. 0.20 moles) (with regard to bromomethyl groups) of the mixed isomers of bis-(bromomethyl)-trimethylbenzene were added all at once. The temperature gradually increased to 75° C. After 1 hour at 70° C.-75° C., G.C. analysis indicated that no starting material remained. One main aldehyde peak was observed with retention times of 7.02 minutes with the DB-1 column and a program of 100° C.-270° C. at 15° C./minute. At 60° C., 28.38 g (0.43 moles) of 50 percent aqueous hydroxylamine was added over about 10 minutes. After about 1.5 hours, the mixture was poured into excess cold water to produce a brown and white sticky semi-solid. The semi-solid was filtered off and let stand for 1 week in the hood to solidify. The product was then recrystallized from a 50/50 isopropanol/water mixture to isolate after filtering and drying, 25.1 g of product. This material was used in the next step of the experiment. The theoretical yield attained as 60.9 percent.

Then, Dinitrile Oxides were prepared From the Mixed Isomers of 1,2,4-Trimethyl-isophthaloaldehyde Dioximes which had been synthesized according to the reaction:

In the procedure used, a mechanical stirrer with a stainless steel agitator blade was set up in a 2-liter stainless steel beaker. Then, 25.1 g (ca. 0.1218 moles) of the crude bis-oxime was dissolved in 667 ml of isopropanol in the beaker with stirring. The beaker was placed in an ice water bath and the solution cooled to 10° C.-15° C. Over a 2-hour period, 400 g (ca. 0.40 moles) of commercial bleach (ca. 5 percent NaCl0) was then slowly added dropwise to the stirred oxime solution. As the product dinitrile oxide forms, it precipitates from the isopropanol mixture. Ten minutes after all the 5 percent sodium hypochlorite has been added, the mixture was diluted with 1 liter of water and then filtered to remove the crude product.

The product was air-dried to yield 19.13 g of a yellow solid. The FTIR spectrum of this material shows a very sharp distinctive absorption band for the —CNO functionality at 2285 cm⁻¹. The crude yield attained was 77.0 percent.

EXAMPLE 4-7

In this series of experiments, Sunigum® P7395 rubber was cured by the process of this invention using mesitylene dinitrile oxide (MDNO) as the curing agent. In the procedure used, various amounts of MDNO, 30 parts by weight of N330 carbon black, 3 parts by weight of polyethylene oxide wax and 2 parts by weight of Wingstay® 29P antioxidant were blended into 100 parts by weight of the Sunigume® P7395 rubber. The blending was carried out in a Laboratory scale Banbury mixer at 80° C. for 2 minutes. Then, the polymer samples were compression-molded at 150° C. for 6 minutes. The Sunigum® P7395 rubber used in these experiments contained repeat units which were derived from n-butyl acrylate, acrylonitrile, methylacrylate, 1,4-butane diol dimethacrylate, styrene and divinylbenzene. The Sunigum® P7395 rubber did not contain carbon-carbon double bonds.

Example 7 was carried out as a control with no MDNO being blended into the Sunigum® P7395 rubber. In Examples 4-6, from 1 phr to 3 phr of MDNO was blended into the acrylic polymer. The physical properties of the samples made are shown in Table I.

TABLE I Example 4 5 6 7 MDNO Level (phr) 1 2 3 0 Comp Set (%) 69 49 42.9 90.9 50% Modulus (MPa) 4.0 4.5 5.0 3.9 100% Modulus (MPa) 6.7 8.4 — 5.2 Stress Max (MPa) 8.2 8.9 8.4 5.9 Strain (%) 131 106 87 161

As can be seen from Table I, the samples that were blended with MDNO had much lower levels of compression set and higher modulus. Lower compression set and higher modulus are characteristics of higher levels of cure. Maximum strain decreases with increasing levels of cure which is also indicative of the samples made in Examples 4-6 being cured. In any case, this series of experiments shows that MDNO can be used to greatly improve the compression set of the nitrile group containing acrylic polymer samples.

EXAPLE 8

In this experiment, a blend was made using a procedure that was identical to the procedure used in Example 5 except for the fact that the blend made was compression-molded at room temperature for 3 days. The cured sample made had a compression set of 42 percent, a 50 percent modulus of 3.6 MPa, a 100 percent modulus of 9.8 MPa and a stress maximum of 104 percent. Thus, the physical properties of the acrylic polymer cured at room temperature in this experiment were similar to those attained in Examples 4-6 that were cured at 150° C. This experiment accordingly shows that stable aryl dinitrile oxides can be used to cure acrylic polymers at room temperature.

While certain representative embodiments and details have been shown for the purpose of illustrating the subject invention, it will be apparent to those skilled in this art that various changes and modifications can be made without departing from the scope of the present invention. 

What is claimed is:
 1. A process for curing an acrylic polymer which comprises distributing an aromatic compound having at least two nitrile oxide groups bonded thereto throughout the acrylic polymer, wherein the acrylic polymer contains nitrile groups, and wherein a bulky group is in at least one ortho position to the nitrile oxide group and wherein the bulky group is a moiety which is larger than a hydrogen atom.
 2. A crosslinked acrylic polymer which is made by the process specified in claim
 1. 3. A process as specified in claim 1 wherein from about 0.1 phr to about 5 phr of the aromatic compound having at least two nitrile oxide groups is distributed into the acrylic polymer.
 4. A process as specified in claim 3 wherein the aromatic compound having at least two nitrile oxide groups bonded thereto is of the structural formula:

wherein R¹, R² and R³ can be the same or different and are selected from alkyl groups, aryl groups, aryl groups, alkaryl groups, halogen atoms, alkoxy groups and nitro groups.
 5. A process as specified in claim 2 wherein the aromatic compound having at least two nitrile oxide groups bonded thereto is of the structural formula:

wherein Z¹, Z², Z³, Z⁴, Z⁵ and Z⁶ represent nitrile oxide groups or hydrogen atoms, with the proviso that two of the members selected from the group consisting of Z¹, Z², Z³, Z⁴, Z⁵ and Z⁶ represent nitrile oxide groups, and with the proviso that four of the members selected from the group consisting of Z¹, Z², Z³, Z⁴, Z⁵ and Z⁶ represent hydrogen atoms.
 6. A process as specified in claim 2 wherein the aromatic compound having at least two nitrile oxide groups bonded thereto has bulky groups in two positions that are ortho to the nitrile oxide groups.
 7. A process as specified in claim 6 wherein the bulky groups in the aromatic compound having at least two nitrile oxide groups bonded thereto are larger than a fluorine atom.
 8. A process as specified in claim 7 wherein the bulky groups are alkyl groups.
 9. A process as specified in claim 8 wherein the bulky groups are methyl groups.
 10. A process as specified in claim 2 wherein the aromatic compound having at least two nitrile oxide groups bonded thereto is mesitylene dinitrile oxide.
 11. A process as specified in claim 2 wherein the acrylic polymer consists of repeat units that are derived from an alkyl acrylate monomer, an alkyl methacrylate monomer, acrylonitrile and a vinyl aromatic monomer.
 12. A process as specified in claim 2 wherein the acrylic polymer does not contain repeat units that are derived from diene monomers.
 13. A process as specified in claim 2 wherein the acrylic polymer does not contain carbon-carbon double bonds.
 14. A process as specified in claim 10 wherein the acrylic polymer does not contain carbon-carbon double bonds; and wherein from about 0.5 phr to about 4 phr of the mesitylene dinitrile oxide is distributed into the acrylic polymer.
 15. A process as specified in claim 14 wherein from about 1 phr to about 3 phr of the mesitylene dinitrile oxide is distributed into the acrylic polymer.
 16. A process as specified in claim 14 wherein the acrylic polymer consists of repeat units that are made by the polymerization of an alkyl acrylate monomer, alkyl methacrylate monomer, acrylonitrile and a vinyl aromatic monomer.
 17. A process as specified in claim 16 wherein the acrylic polymer does not contain repeat units that are made by polymerizing diene monomers.
 18. A process as specified in claim 16 wherein the vinyl aromatic monomer is styrene. 